Illumination system particularly for microlithography

ABSTRACT

The invention concerns an illumination system, particularly for microlithography with wavelengths ≦93 nm, comprising: a primary light source; a first optical component; a second optical component; an image plane; and an exit pupil; wherein said first optical component transforms said primary light source into a plurality of secondary light sources that are imaged by said second optical component in said exit pupil, wherein said first optical component includes a first optical element having a plurality of first raster elements, that are imaged into said image plane, producing a plurality of images being superimposed at least partially on a field in said image plane, wherein said plurality of first raster elements are rectangular, wherein said filed is a segment of an annulus, and wherein said second optical component includes a first field mirror with negative optical power for shaping said field to said segment of said annulus and a second field mirror with positive optical power, wherein each of a plurality of rays intersects said first field mirror with an incidence angel greater than 70° and wherein each of a plurality of rays intersects said second field mirror with an incidence angle of less than 25°.

[0001] The invention concerns an illumination system for wavelengths≦193 nm as well as a projection exposure apparatus with such anillumination system.

[0002] In order to be able to further reduce the structural widths ofelectronic components, particularly in the submicron range, it isnecessary to reduce the wavelengths of the light utilized formicrolithography. Lithography with very deep UV radiation, so called VUV(Very deep UV) lithography or with soft x-ray radiation, so-called EUV(extreme UV) lithography, is conceivable at wavelengths smaller than 193nm, for example.

[0003] An illumination system for a lithographic device, which uses EUVradiation, has been made known from U.S. Pat. No. 5,339,346. For uniformillumination in the reticle plane and filling of the pupil, U.S. Pat.No. 5,339,346 proposes a condenser, which is constructed as a collectorlens and comprises at least 4 pairs of mirror facets, which are arrangedsymmetrically. A plasma light source is used as the light source.

[0004] In U.S. Pat. No. 5,737,137, an illumination system with a plasmalight source comprising a condenser mirror is shown, in which anillumination of a mask or a reticle to be illuminated is achieved bymeans of spherical mirrors.

[0005] U.S. Pat. No. 5,361,292 shows an illumination system, in which aplasma light source is provided, and the point plasma light source isimaged in an annular illuminated surface by means of a condenser, whichhas five aspherical mirrors arranged off-center.

[0006] From U.S. Pat. No. 5,581,605, an illumination system has beenmade known, in which a photon beam is split into a multiple number ofsecondary light sources by means of a plate with concave rasterelements. In this way, a homogeneous or uniform illumination is achievedin the reticle plane. The imaging of the reticle on the wafer to beexposed is produced by means of a conventional reduction optics. Agridded mirror is precisely provided with equally curved elements in theillumination beam path. The contents of the above-mentioned patents areincorporated by reference.

[0007] EP-A-0 939 341 shows an illumination-system and exposureapparatus for illuminating a surface over an illumination field havingan arcuate shape with X-ray wavelength light. The illumination systemcomprises first and second optical integrators each with a plurality ofreflecting elements. The first and second optical integrators beingopposingly arranged such that a plurality of light source images areformed at the plurality of reflecting elements of the second opticalintegrator. To form an arcuate shaped illumination field in the fieldplane according to EP-A-0 939 341 the reflecting elements of the firstoptical integrator have an arcuate shape similar to the arcuateillumination field. Such reflecting elements are complicate tomanufacture.

[0008] EP-A-1 026 547 also shows an illumination system with two opticalintegrators. Similar to the system of EP-A-0 939 341 the reflectingelements of the first optical integrator have an arcuate shape forforming an arcuate shaped illumination field in the field plane.

[0009] In EP-A-0 955 641 a system with two optical integrators is shown.Each of said optical integrators comprises a plurality ofraster-elements. The raster elements of the first optical integrator areof rectangular shape. The arc-shaped field in the field plane is formedby at least one grazing incidence field mirror. The content of the abovementioned patent-application is incorporated by reference. All abovementioned illumination systems have the disadvantage that thetrack-length of the illumination system is large.

[0010] It is therefore an object of the invention to overcome thedisadvantages of the illumination systems according to the state of theart, to provide an illumination system for microlithography thatfulfills the, requirements for advanced lithography with wavelength lessor equal to 193 nm and which is of compact size.

[0011] The object of the invention is solved by an illumination systemwith the features of claim 1 and by an projection exposure apparatusaccording to claim 17.

[0012] The system illuminates a structured reticle arranged in the imageplane of the illumination system, which will be imaged by a projectionobjective onto a light sensitive substrate. In reflective lithographysystems the reticle is illuminated with an arc-shaped field, wherein apregiven uniformity of the scanning energy distribution inside the fieldis required, for example better than ±5%. The scanning energy is definedas the line integral over the light intensity in the scanning direction.A further requirement is the illumination of the exit pupil of theillumination system, which is located at the entrance pupil of theprojection objective. A nearly field-independent illumination of theexit pupil is required.

[0013] Typical light sources for wavelengths between 100 nm and 200 nmare excimer lasers, for example an ArF-Laser for 193 nm, an F₂-Laser for157 nm, an Ar₂-Laser for 126 nm and an NeF-Laser for 109 nm. For systemsin this wavelength region refractive components of SiO₂, CaF₂, BaF₂ orother crystallites are used. Since the transmission of the opticalmaterials deteriorates with decreasing system has to be under vacuum andhence only conduction can be used for cooling.

[0014] Furthermore in an illumination system for lithography it isdesirable to introduce means for cutting off the field e.g. by a fieldstop.

[0015] An illumination system for lithography with a field stop is shownin U.S. Pat. No. 4,294,538. The content of said document Is incorporatedherein fully by reference. The system according to U.S. Pat. No.4,294,538 comprises a slit plate on which an arcuate image of the lightsource is formed. By varying the radial length and the length indirection of the circular arc of the opening of the slit it is possibleto adjust the radial length and the length in the direction of thecircular arc of the arcuate image of the light source on a mask.Therefore the slit plate can also be designated as a field stop. Betweenthe slit plate and the mask there are two mirrors arranged for imagingthe arc-shaped field in the plane of the slit plate onto a reticle-mask.

[0016] Since the illumination system known from U.S. Pat. No. 4,294,538is designed for a light source comprising a ultra high tension mercurylamp emitting light in the visible region the system is totallydifferent to a illumination system for wavelengths ≦193 nm.

[0017] For example said system has no means for enhancing the étendue ofthe light source e.g. by raster elements of a fly's-eye integrator,which is essential for EUV-systems.

[0018] The mirrors according to U.S. Pat. No. 4,294,538 are impinged bythe rays travelling through the system under an angle of 45°, which isnot possible in EUV-systems, since normal incidence mirrors inEUV-systems are comprising more than 40 pairs of alternating layers. Alarge number of alternating layers leads to phase effects if the meanangle of incidence becomes more than ratio in the range of 5:1 and 20:1.The length of the rectangular field raster elements is typically between15 mm and 50 mm, the width is between 1 mm and 4 mm.

[0019] To illuminate an arc-shaped field in the image plane withrectangular field raster elements the second optical component of theillumination system comprises a first field mirror for transforming therectangular images of the rectangular field raster elements toarc-shaped images. The arc length is typically in the range of 80 mm to105 mm, the radial width in the range of 5 mm to 9 mm. Thetransformation of the rectangular images of the rectangular field rasterelements can be done by conical reflection with the first field mirrorbeing a grazing incidence mirror with negative optical power. In otherwords, the imaging of the field raster elements is-distorted to get thearc-shaped images, wherein the radius of the arc is determined by theshape of the object field of the projection objective. The first fieldmirror is preferably arranged in front of the image plane of theillumination system, wherein there should be a free working distance.For a configuration with a reflective reticle the free working distancehas to be adapted to the fact that the rays traveling from the reticleto the projection objective are not vignetted by the first field mirror.

[0020] The surface of the first field mirror is preferably an off-axissegment of a rotational symmetric reflective surface, which can bedesigned aspherical or spherical. The axis of symmetry of the supportingsurface goes through the vertex of the surface. Therefore a segmentaround the vertex is called on-axis, wherein each segment of thesurfaces which does not include the vertex is called off-axis. Thesupporting surface can be manufactured more easily due to the rotationalsymmetry. After producing the supporting surface the segment can be cutout with well-known techniques.

[0021] The surface of the first field mirror can also be designed as anon-axis segment of a toroidal reflective surface. Therefore the surfacehas to be processed locally, but has the advantage that the surroundingshape can be produced before surface treatment.

[0022] The incidence angles of the incoming rays with respect to thesurface normals at the points of incidence of the incoming rays on thefirst field mirror are preferably greater than 70°, which results in areflectivity of the first field mirror of more than 80%.

[0023] The second optical component comprises a second field mirror withpositive optical power. The first and second field mirror together imagethe secondary light sources or the pupil plane respectively into theexit pupil of the illumination system, which is defined by the entrancepupil of the projection objective. The second field mirror is arrangedbetween the plane with the secondary light sources and the first fieldmirror.

[0024] The second field mirror is preferably an off-axis segment of arotational symmetric reflective surface, which can be designedaspherical or spherical, or an on-axis segment of a toroidal reflectivesurface.

[0025] The incidence angles of the incoming rays with respect to thesurface normals at the points of incidence of the incoming rays on thesecond field mirror are preferably lower than 25°. Since the mirrorshave to be coated with multilayers for the EUV wavelength region, thedivergence and the incidence angles of the incoming rays are preferablyas low as possible to increase the reflectivity, which should be betterthan 65%.

[0026] To reduce the length of the illumination system the field lenscomprises preferably a third field mirror. The third field mirror ispreferably arranged between the plane with the secondary light sourcesand the second field mirror.

[0027] The third field mirror has preferably negative optical power andforms together with the second and first field mirror an opticaltelescope system having a object plane at the secondary light sourcesand an image plane at the exit pupil of the illumination system to imagethe secondary light sources into the exit pupil. The pupil plane of thetelescope system is arranged at the image plane of the illuminationsystem. Therefore the ray bundles coming from the secondary lightsources are superimposed in the pupil plane of the telescope system orin the image plane of the illumination system accordingly. The firstfield mirror has mainly the function of forming the arc-shaped field,wherein the telescope system is mainly determined by the negative thirdfield mirror and the positive second field mirror.

[0028] In another embodiment the third field mirror has preferablypositive optical power to generate images of the secondary light sourcesin a plane between the third and second field mirror, forming tertiarylight sources. The tertiary light sources are imaged with the secondfield mirror and the first field mirror into the exit pupil of theillumination system. The images of the tertiary light sources in theexit pupil of the illumination system are called in this case quaternarylight sources.

[0029] Since the plane with the tertiary light sources is arrangedconjugated to the exit pupil, this plane can be used to arrange maskingblades to change the illumination mode or to add transmission filters.This position in the beam path has the advantage to be freelyaccessible.

[0030] The third field mirror is similar to the second field mirrorpreferably an off-axis segment of a rotational symmetric reflectivesurface, which can be designed aspherical or spherical, or an on-axissegment of a toroidal reflective surface.

[0031] The incidence angles of the incoming rays with respect to thesurface normals at the points of incidence of the incoming rays on thethird field mirror are preferably lower than 25°. With the third fieldmirror being arranged as a normal incidence mirror the beam path can befolded again to reduce the overall size of the illumination system.

[0032] To avoid vignetting of the beam path the first, second and thirdfield mirrors are preferably arranged in a non-centered system. There isno common axis of symmetry for the mirrors. An optical axis can bedefined as a connecting line between the centers of the used areas onthe field mirrors, wherein the optical axis is bent at the field mirrorsdepending on the tilt angles of the field mirrors.

[0033] It is advantageous to insert a second optical element with secondraster elements in the light path after the first optical element withfirst raster elements, wherein one first raster element corresponds toone of the second raster elements. In this case deflection angles of thefirst raster elements are designed to deflect the ray bundles impingingon the first raster elements to the corresponding second rasterelements.

[0034] The second raster elements are preferably arranged at thesecondary light sources and are designed to image together with thefield lens the first raster elements or field raster elements into theimage plane of the illumination system, wherein the images of the fieldraster elements are at least partially superimposed. The second rasterelements are called pupil raster elements or pupil honeycombs.

[0035] With the tilt angles' of the reflective components of theillumination system the beam paths between the components can be bent.Therefore the orientation of the beam cone emitted by the source and theorientation of the image plane system can be arranged according to therequirements of the overall system. A preferable configuration has asource emitting a beam cone in one direction and an image plane having asurface normal which is oriented almost perpendicular to this direction.In one embodiment the source emits Horizontally and the image plane hasa vertical surface normal. Some light sources like undulator or wigglersources emit only in the horizontal plane. On the other hand the reticleshould be arranged horizontally for gravity reasons. The beam paththerefore has to be bent between the light source and the image planeabout almost 90°. Since mirrors with incidence angles between 30° and60° lead to polarization effects and therefore to light losses, the beambending has to be done only with grazing incidence or normal incidencemirrors. For efficiency reasons the number of mirrors has to be as smallas possible.

[0036] By definition all rays intersecting the field in the image planehave to go through the exit pupil of the illumination system. Theposition of the field and the position of the exit pupil are defined bythe object field and the entrance pupil of the projection objective. Forsome projection objectives being centered systems the object field isarranged off-axis of an optical axis, wherein the entrance pupil isarranged on-axis in a finite distance to the object plane. For theseprojection objectives an angle between a straight line from the centerof the object field to the center of the entrance pupil and the surfacenormal of the object plane can be defined. This angle is in the range of3° to 10° for EUV projection objectives. Therefore the components of theillumination system have to be configured and arranged in such a waythat all rays intersecting the object field of the projection objectiveare going through the entrance pupil of the projection objective beingdecentered to the object field. For projection exposure apparatus with areflective reticle all rays intersecting the reticle needs to haveincidence angles greater than 0° to avoid vignetting of the reflectedrays at components of the illumination system.

[0037] In the EUV wavelength region all components are reflectivecomponents, which are arranged preferably in such a way, that allincidence angles on the components are lower than 25° or greater than65°. Therefore polarization effects arising for incidence angles aroundan angle of 45° are minimized. Since grazing incidence mirrors have areflectivity greater than 80%, they are preferable in the optical designin comparison to normal incidence mirrors with a reflectivity greaterthan 65%.

[0038] The illumination system is typically arranged in a mechanicalbox. By folding the beam path with mirrors the overall size of the boxcan be reduced. This box preferably does not interfere with the imageplane, in which the reticle and the reticle supporting system arearranged. Therefore it is advantageous to arrange and tilt thereflective components in such a way that all components are completelyarranged on one side of the reticle. This can be achieved if the fieldlens comprises only an even number of normal incidence mirrors.

[0039] The illumination system as described before can be usedpreferably in a projection exposure apparatus comprising theillumination system, a reticle arranged in the image plane of theillumination system and a projection objective to image the reticle ontoa wafer arranged in the image plane of the projection objective. Both,reticle and wafer are arranged on a support unit, which allows theexchange or scan of the reticle or wafer.

[0040] The projection objective can be a catadioptric lens, as knownfrom U.S. Pat. No. 5,402,267 for wavelengths in the range between 100 nmand 200 nm. These systems have typically a transmission reticle.

[0041] For the EUV wavelength range the projection objectives arepreferably all-reflective systems with four to eight mirrors as knownfor example from U.S. Ser. No. 09/503,640 showing a six mirrorprojection lens. These systems have typically a reflective reticle.

[0042] For systems with a reflective reticle the illumination beam pathbetween the light source and the reticle and the projection beam pathbetween the reticle and the wafer preferably interfere only nearby thereticle, where the incoming and reflected rays for adjacent objectpoints are traveling in the same region. If there are no furthercrossing of the illumination and projection beam path it is possible toseparate the illumination system and the projection objective except forthe reticle region.

[0043] The projection objective has preferably a projection beam pathbetween said reticle and the first imaging element which is tiltedtoward the optical axis of the projection objective. Especially for aprojection exposure apparatus with a reflective reticle the separationof the illumination system and the projection objective is easier toachieve.

[0044] The invention will be described below on the basis of drawings.

[0045] Here:

[0046]FIG. 1: A schematic view of a first embodiment with convex mirrorsas field raster elements and planar mirrors as pupil raster elements

[0047]FIG. 2: A schematic view of a second embodiment with convexmirrors as field raster elements and concave mirrors as pupil rasterelements.

[0048]FIG. 3: A schematic view of the principal setup of an illuminationsystem.

[0049]FIG. 4: An Arrangement of the field raster elements.

[0050]FIG. 5: An Arrangement of the pupil raster elements.

[0051]FIG. 6: A schematic view of a third embodiment with a concavepupil-imaging field mirror and a convex field-forming field mirror.

[0052]FIG. 7: A schematic view of a further embodiment with a secondoptical component comprising a telescope system and a convexfield-forming field mirror.

[0053]FIG. 8: A detailed view of the embodiment of FIG. 7.

[0054]FIG. 9: Intensity distribution of the embodiment of FIG. 7.

[0055]FIG. 10: Illumination of the exit pupil of the illumination systemof the embodiment of FIG. 7.

[0056]FIG. 11: A schematic view of a embodiment with an intermediateimage of the light source.

[0057]FIG. 12: A detailed view of a projection exposure apparatus.

[0058]FIG. 1 shows an first embodiment of the invention in aschematically view. The beam cone of the light source 7001 is collectedby the ellipsoidal collector mirror 7003 and is directed to the platewith the field raster elements 7009. The collector mirror 7003 isdesigned to generate an image 7005 of the light source 7001 between theplate with the field raster elements 7009 and the plate with the pupilraster elements 7015 if the plate with the field raster elements 7009would be a planar mirror as indicated by the dashed lines. The convexfield raster elements 7009 are designed to generate point-like secondarylight sources 7007 at the pupil raster elements 7015, since the lightsource 7001 is also point-like. Therefore the pupil raster elements 7015are designed as planar mirrors. Since the intensity at the point-likesecondary light sources 7007 is very high, the planar pupil rasterelements 7015 can alternatively be arranged defocused from the secondarylight sources 7007. The distance between the secondary light sources7007 and the pupil raster elements 7015 should not exceed 20% of thedistance between the field raster elements and the pupil rasterelements. The pupil raster elements 7015 are tilted to superimpose theimages of the field raster elements 7009 together with the field lens7021 formed as the field mirrors 7023 and 7027 in the field 7031 to beilluminated. Both, the field raster elements 7009 and the pupil rasterelements 7015 are tilted. Therefore the assignment between the fieldraster elements 7009 and pupil raster elements 7015 is defined by theuser. In the embodiment of FIG. 1 the field raster elements at thecenter of the plate with the field raster elements 7009 correspond tothe pupil raster elements at the border of the plate with the pupilraster elements 7015 and vice versa. The tilt angles and the tilt axesof the field raster elements are determined by the directions of theincoming ray bundles and by the positions of the corresponding pupilraster elements 7015. Since for each field raster element 7009 the tiltangle and the tilt axis is different, also the planes of incidencedefined by the incoming and reflected centroid rays are not parallel.The tilt angles and the tilt axes of the pupil raster elements 7015 aredetermined by the positions of the corresponding field raster elements7009 and the requirement that the images of the field raster elements7009 have to be superimposed in the field 7031 to be illuminated. Theconcave field mirror 7023 images the secondary light sources 7007 intothe exit pupil 7033 of the illumination system forming tertiary lightsources 7035, wherein the convex field mirror 7027 being arranged atgrazing incidence transforms the rectangular images of the rectangularfield raster elements 7009 into arc-shaped images.

[0059]FIG. 2 shows second embodiment in a schematically view.Corresponding elements have the same reference numbers as-those in FIG.1 increased by 100. Therefore, the description to these elements isfound in the description to FIG. 1. In this embodiment the light source7101 and therefore also the secondary light sources 7107 are extended.The pupil raster elements 7115 are designed as concave mirrors to imagethe field raster elements 7109 into the image plane 7129. It is alsopossible to arrange the pupil raster elements 7115 not at the secondarylight sources 7107, but defocused. The influence of the defocus on theimaging of the field raster elements 7109 has to be considered in theoptical power of the pupil raster elements.

[0060]FIG. 3 shows in a schematic view the imaging of one field rasterelement 7209 into the reticle plane 7229 forming an image 7231 and theimaging of the corresponding secondary light source 7207 into the exitpupil 7233 of the illumination system forming a tertiary light source7235. Corresponding elements have the same reference numbers as those inFIG. 1 increased by 200. Therefore, the description to these elements isfound in the description to FIG. 1.

[0061] The field raster elements 7209 are rectangular and have a lengthX_(FRE) and a width Y_(FRE). All field raster elements 7209 are arrangedon a nearly circular plate with a diameter D_(FRE). They are imaged intothe image plane 7229 and superimposed on a field 7231 with a lengthX_(field) and a width Y_(field), wherein the maximum aperture in theimage plane 7229 is denoted by NA_(field). The field size corresponds tothe size of the object field of the projection objective, for which theillumination system is adapted to.

[0062] The plate with the pupil raster elements 7215 is arranged in adistance of Z₃ from the plate with the field raster elements 7209. Theshape of the pupil raster elements 7215 depends on the shape of thesecondary light sources 7207. For circular secondary light sources 7207the pupil raster elements 7215 are circular or hexagonal for a densepackaging of the pupil raster elements 7215. The diameter of the platewith the pupil raster elements 7215 is denoted by D_(PRE).

[0063] The pupil raster elements 7215 are imaged by the second opticalcomponent, which is depicted in FIG. 3 as a field lens 7221 into theexit pupil 7233 having a diameter of D_(EP). The distance between theimage plane 7229 of the illumination system and the exit pupil 7233 isdenoted with Z_(EP). Since the exit pupil 7233 of the illuminationsystem corresponds to the entrance pupil of the projection objective,the distance Z_(EP) and the diameter D_(EP) are predetermined values.The entrance pupil of the projection objective is typically illuminatedup to a user-defined filling ratio σ.

[0064] The data for a preliminary design of the illumination system canbe calculated with the equations and data given below. The values forthe parameters are typical for a EUV projection exposure apparatus. Butthere is no limitation to these values. Wherein the schematic design isshown for a refractive linear system it can be easily adapted forreflective systems by exchanging the lenses with mirrors.

[0065] The field 7231 to be illuminated is defined by a segment of anannulus. The Radius of the annulus is

[0066] R_(field)=138 mm.

[0067] The length and the width of the segment are

[0068] X_(field)=88 mm, Y_(field)=8 mm

[0069] Without the field-forming field mirror of the second opticalcomponent which transforms the rectangular images of the field rasterelements into arc-shaped images the field to be illuminated isrectangular with the length and width defined by the segment of theannulus.

[0070] The distance from the image plane to the exit pupil is

[0071] Z_(EP)=1320 mm.

[0072] The object field of the projection objective is an off-axisfield. The distance between the center of the field and the optical axisof the projection objective is given by the radius R_(field). Thereforethe incidence angle of the centroid ray in the center of the field is6°.

[0073] The aperture at the image plane of the projection objective isNA_(wafer)=0.25. For a reduction projection objective with amagnification ratio of β_(proj)=−0.25 and a filling ratio of σ=0.8 theaperture at the image plane of the illumination system is${NA}_{field} = {{\sigma \cdot \frac{{NA}_{wafer}}{4}} = 0.05}$

 D _(FP)=2 tan └arcsin(NA _(field))┘·Z _(EP)≈2NA _(EP) ·Z _(EP)≈132 mm

[0074] The distance Z₃ between the field raster elements and the pupilraster elements is related to the distance Z_(EP) between the imageplane and the exit pupil by the depth magnification α:

Z _(EP) =α·Z ₃

[0075] The size of the field raster elements is related to the fieldsize by the lateral magnification β_(field):

X _(field)=β_(field) ·X _(FRE)

Y _(field)=β_(field) ·Y _(FRE)

[0076] The diameter D_(PRE) of the plate with the pupil raster elementsand the diameter D_(EP) of the exit pupil are related by the lateralmagnification β_(pupil):

D _(EP)=β_(pupil) ·D _(PRE)

[0077] The depth magnification a is defined by the product of thelateral magnifications β_(field) and β_(pupil):

α=β_(field)·β_(pupil)

[0078] The number of raster elements being superimposed at the field isset to 200.

[0079] With this high number of superimposed images the required fieldillumination uniformity can be achieved.

[0080] Another requirement is to minimize the incidence angles on thecomponents. For a reflective system the beam path is bent at the platewith the field raster elements and at the plate with the pupil rasterelements. The bending angles and therefore the incidence angles areminimum for equal diameters of the two plates:

D_(PRE)=D_(FRE)${200 \cdot X_{PRE} \cdot Y_{PRE}} = {{200 \cdot \frac{X_{field} \cdot Y_{field}}{\beta_{field}^{2}}} = {\frac{D_{EP}^{2}}{\beta_{pupil}^{2}} = {\frac{\beta_{field}^{2}}{\alpha^{2}}D_{EP}^{2}}}}$

[0081] The distance Z₃ is set to Z₃=900 mm. This distance is acompromise between low incidence angles and a reduced overall length ofthe illumination system. $\alpha = {\frac{Z_{EP}}{Z_{3}} = 1.47}$Therefore${\beta_{field}} \approx \sqrt[4]{\frac{200 \cdot X_{field} \cdot Y_{field}}{D_{EP}^{2}}\alpha^{2}} \approx 2.05$${\beta_{pupil}} \approx \frac{\alpha}{\beta_{field}} \approx 0.7$$D_{FRE} = {D_{PRE} = {{\frac{\beta_{field}}{\alpha}D_{EP}} \approx {200\quad {mm}}}}$$X_{FRE} = {\frac{X_{field}}{\beta_{field}} \approx {43\quad {mm}}}$$Y_{FRE} = {\frac{Y_{field}}{\beta_{field}} \approx {4\quad {mm}}}$

[0082] With these values the principal layout of the illumination systemis known.

[0083] In a next step the field raster elements 7309 have to bedistributed on the plate as shown in FIG. 4. The two-dimensionalarrangement of the field raster elements 7309 is optimized forefficiency. Therefore the distance between the field raster elements7309 is as small as possible. Field raster elements 7309, which are onlypartially illuminated, will lead to uniformity errors of the intensitydistribution in the image plane, especially in the case of a restrictednumber of field raster elements 7309. Therefore only these field rasterelements 7309 are imaged into the image plane which are illuminatedalmost completely. FIG. 4 shows a possible arrangement of 216 fieldraster elements 7309. The solid line 7339 represents the border of thecircular illumination of the plate with the field raster elements 7309.Therefore the filling efficiency is approximately 90%. The rectangularfield raster elements 7309 have a length X_(FRE)=46.0 mm and a widthY_(FRE)=2.8 mm. All field raster elements 7309 are inside the circle7339 with a diameter of 200 mm. The field raster elements 7309 arearranged in 69 rows 7341 being arranged one among another. The fieldraster elements 7309 in the rows 7341 are attached at the smaller y-sideof the field raster elements 7309. The rows 7341 consist of one, two,three or four field raster elements 7309. Some rows 7341 are displacedrelative to the adjacent rows 7341 to distribute the field rasterelements 7309 inside the circle 7339. The distribution is symmetrical tothe y-axis.

[0084]FIG. 5 shows the arrangement of the pupil raster elements 7415.They are arranged on a distorted grid to compensate for distortionerrors of the field lens. If this distorted grid of pupil rasterelements 7415 is imaged into the exit pupil of the illumination systemby the field lens an undistorted regular grid of tertiary light sourceswill be generated. The pupil raster elements 7415 are arranged on curvedlines 7443 to compensate the distortion introduced by the field-formingfield mirror. The distance between adjacent pupil raster elements 7415is increased in y-direction to compensate the distortion introduced byfield mirrors being tilted about the x-axis. Therefore the pupil rasterelements 7415 are not arranged inside a circle. The size of the pupilraster elements 7415 depends on the source size or source étendue. Ifthe source étendue is much smaller than the required étendue in theimage plane, the secondary light sources will not fill the plate withthe pupil raster elements 7415 completely. In this case the pupil rasterelements 7415 need only to cover the area of the secondary light sourcesplus some overlay to compensate for source movements and imagingaberrations of the collector-field raster element unit. In FIG. 5circular pupil raster elements 7415 are shown.

[0085] Each field raster element 7309 correspond to one of the pupilraster elements 7415 according to a assignment table and is tilted todeflect an incoming ray bundle to the corresponding pupil raster element7415. A ray coming from the center of the light source and intersectingthe field raster element 7309 at its center is deflected to intersectthe center of the corresponding pupil raster element 7415. The tiltangle and tilt axis of the pupil raster element 7415 is designed todeflect this ray in such a way, that the ray intersects the field in itscenter.

[0086] The second optical component comprising the field mirror imagesthe plate with the pupil raster elements into the exit pupil andgenerates the arc-shaped field with the desired radius R_(field). ForR_(field)≈=138 mm, the field forming gracing incidence field mirror hasonly low negative optical power. The optical power of the field-formingfield mirror has to be negative to get the correct orientation of thearc-shaped field. Since the magnification ratio of the second opticalcomponent has to be positive, another field mirror with positive opticalpower is required. The field mirror with positive optical power is anormal incidence mirror. The usage of a normal incidence mirror providesfor a compact size of the illumination system.

[0087]FIG. 6 shows a schematic view of a embodiment comprising a lightsource 7501, a collector mirror 7503, a plate with the field rasterelements 7509, a plate with the pupil raster elements 7515, a field lens7521, a image plane 7529 and a exit pupil 7535. The field lens 7521 hasone normal-incidence mirror 7523 with positive optical power for pupilimaging and one grazing-incidence mirror 7527 with negative opticalpower for field shaping. Exemplary for the imaging of all secondarylight sources, the imaging of one secondary light source 7507 into theexit pupil 7533 forming a tertiary light source 7535 is shown. Theoptical axis 7545 of the illumination system is not a straight line butis defined by the connection lines between the single components beingintersected by the optical axis 7545 at the centers of the components.Therefore, the illumination system is a non-centered system having anoptical axis 7545 being bent at each component to get a beam path freeof vignetting. There is no common axis of symmetry for the opticalcomponents. Projection objectives for EUV exposure apparatus aretypically centered systems with a straight optical axis and with anoff-axis object field. The optical axis 7547 of the projection objectiveis shown as a dashed line. The distance between the center of the field7531 and the optical axis 7547 of the projection objective is equal tothe field radius R_(field).

[0088] In another embodiment as shown in FIG. 7, a telescope objectivein the field lens 7621 comprising the field mirror 7623 with positiveoptical power, the field mirror 7625 with negative optical power and thefield mirror 7627 is applied to reduce the track length. Correspondingelements have the same reference numbers as those in FIG. 6 increased by100. Therefore, the description to these elements is found in thedescription to FIG. 6. The field mirror 7625 and the field mirror 7623of the telescope objective in FIG. 5 are formed as an off-axisCassegrainian configuration. The telescope objective has an object planeat the secondary light sources 7607 and an image plane at the exit pupil7633 of the illumination system. The pupil plane of the telescopeobjective is arranged at the image plane 7629 of the illuminationsystem. In this configuration, having five normal-incidence reflectionsat the mirrors 7603, 7609, 7615, 7625 and 7623 and one grazing-incidencereflection at the mirror 7627, all mirrors are arranged below the imageplane 7629 of the illumination system. Therefore, there is enough spaceto install the reticle and the reticle support system.

[0089] In FIG. 8 a detailed view of the embodiment of FIG. 7 is shown.Corresponding elements have the same reference numbers as those in FIG.7 increased by 100. Therefore, the description to these elements isfound in the description to FIG. 7. The components are shown in ay-z-sectional view, wherein for each component the local co-ordinatesystem with the y- and z-axis is shown. For the collector mirror 7703and the field mirrors 7723, 7725 and 7727 the local coordinate systemsare defined at the vertices of the mirrors. For the two plates with theraster elements the local co-ordinate systems are defined at the centersof the plates. In table 2 the arrangement of the local co-ordinatesystems with respect to the local co-ordinate system of the light source7701 is given. The tilt angles α, β and γ about the x-, y- and z-axisare defined in a right-handed system. TABLE 2 Co-ordinate systems ofvertices of mirrors X[mm] Y[mm] Z[mm] α[°] β[°] γ[°] Light source 77010.0 0.0 0.0 0.0 0.0 0.0 Collector mirror 0.0 0.0 125.0 0.0 0.0 0.0 7703Plate with field 0.0 0.0 −975.0 10.5 180.0 0.0 raster elements 7709Plate with pupil 0.0 −322.5 −134.8 13.5 0.0 180.0 raster elements 7715Field mirror 7725 0.0 508.4 −1836.1 −67.8 0.0 180.0 Field mirror 77230.0 204.8 −989.7 −19.7 0.0 180.0 Field mirror 7727 0.0 −163.2 −2106.249.4 180.0 0.0 Image plane 7731 0.0 −132.1 −1820.2 45.0 0.0 0.0 Exitpupil 7733 0.0 −1158.1 −989.4 45.0 0.0 0.0

[0090] The surface data are given in table 3. The radius R and theconical constant K define the surface shape of the mirrors according tothe formula${z = {\frac{\rho \quad h^{2}}{1 + \sqrt{1 - {\left( {1 + \kappa} \right)\rho^{2}h^{2}}}} + {\sum\limits_{k = 1}{c_{k}h^{{2k} + 2}}}}},$

[0091] wherein h is the radial distance of a surface point from thez-axis. TABLE 3 Optical data of the components Field Collector rastermirror element Pupil raster Field Field Field 7703 7709 element 7715mirror 7725 mirror 7723 mirror 7727 R [mm] −235.3 ∞ −1239.7 −534.7−937.7 −65.5 K −0.77855 0.0 0.0 −0.0435 −0.0378 −1.1186 Focal — ∞ 617.6−279.4 477.0 −757.1 length f [mm]

[0092] The light source 7701 in this embodiment is aLaser-Produced-Plasma source having a diameter of approximately 0.3 mmgenerating a beam cone with an opening angle of 83°. To decrease thecontamination of the collector mirror 7703 by debris of the source 7701the distance to the collector mirror 7703 is set to 125 mm.

[0093] The collector mirror 7703 is an elliptical mirror, wherein thelight source 7701 is arranged in the first focal point of the ellipsoidand wherein the plate with the pupil raster elements 7715 is arranged inthe second focal point of the ellipsoid.

[0094] Therefore the field raster elements 7709 can be designed asplanar mirrors. The distance between the vertex of the collector mirror7703 and the center of the plate with the field raster elements 7709 is1100 mm. The field raster elements 7709 are rectangular with a lengthX_(FRE)=46.0 mm and a width Y_(FRE)=2.8 mm. The arrangement of the fieldraster elements is shown in FIG. 4. The tilt angles and tilt axis aredifferent for each field raster element 7709, wherein the field rasterelements are tilted to direct the incoming ray bundles to thecorresponding pupil raster elements 7715. The tilt angles are in therange of −4° to 4°. The mean incidence angle of the rays on the fieldraster elements is 10.5°. Therefore the field raster elements 7709 areused at normal incidence.

[0095] The plate with the pupil raster elements 7715 is arranged in adistance of 900 mm from the plate with the field raster elements 7709.The pupil raster elements 7715 are concave mirrors. The arrangement ofthe pupil raster elements 7715 is shown in FIG. 5. The tilt angles andtilt axis are different for each pupil raster element 7715, wherein thepupil raster elements 7715 are tilted to superimpose the images of thefield raster elements 7709 in the image plane 7731. The tilt angles arein the range of −4° to 4°. The mean incidence angle of the rays on thepupil raster elements 7715 is 7.5°. Therefore the pupil raster elements7715 are used at normal incidence.

[0096] The field mirror 7725 is a convex mirror. The used area of thismirror defined by the incoming rays is an off-axis segment of arotational symmetric conic surface. The mirror surface is drawn in FIG.6 from the vertex up to the used area as dashed line. The distancebetween the center of the plate with the pupil raster elements 7715 andthe center of the used area on the field mirror 7725 is 1400 mm. Themean incidence angle of the rays on the field mirror 7725 is 12°.Therefore the field mirror 7725 is used at normal incidence.

[0097] The field mirror 7723 is a concave mirror. The used area of thismirror defined by the incoming rays is an off-axis segment of arotational symmetric conical surface. The mirror surface is drawn inFIG. 75 from the vertex up to the used area as dashed line. The distancebetween the center of the used area on the field mirror 7725 and thecenter of the used area on the field mirror 7723 is 600 mm. The meanincidence angle of the rays on the field mirror 7723 is 7.5°. Thereforethe field mirror 7723 is used at normal incidence.

[0098] The field mirror 7727 is a convex mirror. The used area of thismirror defined by the incoming rays is an off-axis segment of arotational symmetric conic surface. The mirror surface is drawn in FIG.6 from the vertex up to the used area as dashed line. The distancebetween the center of the used area on the field mirror 7723 and thecenter of the used area on the field mirror 7727 is 600 mm. The meanincidence angle of the rays on the field mirror 7727 is 78°. Thereforethe field mirror 7727 is used at grazing incidence. The distance betweenthe field mirror 7727 and the image plane 7731 is 300 mm.

[0099] In another embodiment the field mirror and the field mirror arereplaced with on-axis toroidal mirrors. The vertices of these mirrorsare arranged in the centers of the used areas. The convex field mirrorhas a radius R_(y)=571.3 mm in the y-z-section and a radius R_(x)=546.6mm in the x-z-section. This mirror is tilted about the local x-axisabout 12° to the local optical axis 7745 defined as the connection linesbetween the centers of the used areas of the mirrors. The concave fieldmirror has a radius R_(y)=−962.14 mm in the y-z-section and a radiusR_(x)=−945.75 mm in the x-z-section. This mirror is tilted about thelocal x-axis about 7.5° to the local optical axis 7745.

[0100]FIG. 9 shows the illuminated arc-shaped area in the image plane7731 of the illumination system presented in FIG. 8. The orientation ofthe y-axis is defined in FIG. 8. The solid line 7849 represents the50%-value of the intensity distribution, the dashed line 7851 the10%-value. The width of the illuminated area in y-direction is constantover the field. The intensity distribution is the result of a simulationdone with the optical system given in table 2 and table 3.

[0101]FIG. 10 shows the illumination of the exit pupil 7733 for anobject point in the center (x=0 mm; y=0 mm) of the illuminated field inthe image plane 7731. The arrangement of the tertiary light sources 7935corresponds to the arrangement of the pupil raster elements 7715, whichis presented in FIG. 5. Wherein the pupil raster elements in FIG. 5 arearranged on a distorted grid, the tertiary light sources 7935 arearranged on a undistorted regular grid. It is obvious in FIG. 10, thatthe distortion errors of the imaging of the secondary light sources dueto the tilted field mirrors and the field-shaping field mirror arecompensated. The shape of the tertiary light sources 7935 is notcircular, since the light distribution in the exit pupil 7733 is theresult of a simulation with a Laser-Plasma-Source which was notspherical but ellipsoidal. The source ellipsoid was oriented in thedirection of the local optical axis. Therefore also the tertiary lightsources are not circular, but elliptical. Due to the mixing of the lightchannels and the user-defined assignment between the field rasterelements and the pupil raster elements, the orientation of the tertiarylight sources 7935 is different for each tertiary light source 7935.

[0102] Due to the mixing of the light channels and the user-definedassignment between the field raster elements and the pupil rasterelements, the orientation of the tertiary light sources 7935 isdifferent for nearby each tertiary light source 7935. Therefore, theplanes of incidence of at least two field raster elements have tointersect each other. The plane of incidence of a field raster elementis defined by the centroid ray of the incoming bundle and itscorresponding deflected ray.

[0103]FIG. 11 shows another embodiment in a schematic view.Corresponding elements have the same reference numbers as those in FIG.6 increased by 800. Therefore, the description to these elements isfound in the description to FIG. 6. In this embodiment the collectormirror 8303 is designed to generate an intermediate image 8361 of thelight source 8301 in front of the plate with the field raster elements8309. Nearby this intermediate image 8363 a transmission plate 8365 isarranged. The distance between the intermediate image 8363 and thetransmission plate 8365 is so large that the plate 8365 will not bedestroyed by the high intensity near the intermediate focus. Thedistance is limited by the maximum diameter of the transmission plate8365 which is in the order of 200 mm. The maximum diameter is determinedby the possibility to manufacture a plate being transparent at EUV. Thetransmission plate 8365 can also be used as a spectral purity filter toselect the used wavelength range. Instead of the absorptive transmissionplate 8365 also a reflective grating filter can be used. The plate withthe field raster elements 8309 is illuminated with a diverging raybundle. Since the tilt angles of the field raster elements 8309 areadjusted according to a collecting surface the diverging beam path canbe transformed to a nearly parallel one. Additionally, the field rasterelements 8309 are tilted to deflect the incoming ray bundles to thecorresponding pupil raster elements 8315.

[0104]FIG. 12 shows an EUV projection exposure apparatus in a detailedview. The illumination system is the same as shown in detail in FIG. 8.Corresponding elements have the same reference numbers as those in FIG.8 increased by 700. Therefore, the description to these elements isfound in the description to FIG. 8. In the image plane 8429 of theillumination system the reticle 8467 is arranged. The reticle 8467 ispositioned by a support system 8469. The projection objective 8471having six mirrors images the reticle 8467 onto the wafer 8473 which isalso positioned by a support system 8475. The mirrors of the projectionobjective 8471 are centered on a common straight optical axis 8447. Thearc-shaped object field is arranged off-axis. The direction of the beampath between the reticle 8467 and the first mirror 8477 of theprojection objective 8471 is tilted to the optical axis 8447 of theprojection objective 8471. The angles of the chief rays 8479 withrespect to the normal of the reticle 8467 are between 5° and 7°. Asshown in FIG. 80 the illumination system 8479 is well separated from theprojection objective 8471. The illumination and the projection beam pathinterfere only nearby the reticle 8467. The beam path of theillumination system is folded with reflection angles lower than 25° orhigher than 75° in such a way that the components of the illuminationsystem are arranged between the plane 8481 with the reticle 8467 and theplane 8383 with the wafer 8473.

1. Illumination system, particularly for microlithography withwavelengths 193 nm, comprising: a primary light source (7001, 7101,7501, 7601, 7701, 8301, 8401); a first optical component; a secondoptical component (7021, 7121, 7521, 7621, 8321, 8421); an image plane(7029, 7129, 7229, 7529, 7629, 8329, 8429); and an exit pupil (7033,7133, 7533, 7633, 7733, 8333); wherein said first optical componenttransforms said primary light source (7001, 7101, . . . ) into aplurality of secondary light sources (7007, 7107, 7207) that are imagedby said second optical component (7021, . . . ) in said exit pupil(7033, . . . ), wherein said first optical component includes a firstoptical element having a plurality of first raster elements (7009, 7109,7209, 7309, 7609, 7709, 8309, 8409), that are imaged into said imageplane, producing a plurality of images being superimposed at leastpartially on a field (7031, 7131, 7231, 7531, 7631, 7731, 8331) in saidimage plane (7029, . . . ), wherein said plurality of first rasterelements (7009, . . . ) are rectangular, wherein said field (7031, . . .) is a segment of an annulus, and wherein said second optical componentincludes a first field mirror (7027, 7127, 7527, 7627, 7727, 8327, 8427)with negative optical power for shaping said field to said segment ofsaid annulus and a second field mirror (7023, 7123, 7523, 7623, 7723,8323, 8423) with positive optical power, wherein each of a plurality ofrays intersects said first field mirror (7027, . . . ) with an incidenceangle greater than 70° and wherein each of a plurality of raysintersects said second field mirror (7023, . . . ) with an incidenceangle of less than 25°.
 2. The illumination system according to claim 1,wherein said first field mirror (7027, . . . ) is an off-axis segment ofa rotational symmetric reflective surface.
 3. The illumination systemaccording to claim 1, wherein said first field mirror (7027, . . . ) isan on-axis segment of a toroidal reflective surface.
 4. The illuminationsystem according to one of the claims 1 to 3, wherein said second fieldmirror (7023, . . . ) is an off-axis segment of a rotational symmetricreflective surface.
 5. The illumination system according to one of theclaims 1 to 3, wherein said second field mirror (7023, . . . ) is anon-axis segment of a toroidal reflective surface.
 6. The illuminationsystem according to one of the claims 1 to 5, wherein said secondoptical component comprises a third field mirror (7625, 7725, 8325,8425).
 7. The illumination system according to claim 6, wherein saidthird mirror (7625, . . . ) has negative optical power.
 8. Theillumination system according to claim 6, wherein said first (7027, . .. ), second (7023, . . . ) and third (7625, . . . ) field mirrors form(a) telescope-objective with an tele-object plane at said plurality ofsecondary light sources, (b) a tele-pupil plane at said image plane ofsaid illumination system and (c) a tele-image plane at said exit pupil.9. The illumination system according to one of the claims 6 to 8,wherein each of a plurality of rays intersects said third field mirror(7625, . . . ) with an incidence angle less than 25°.
 10. Theillumination system according to one of the claims 6 to 9, wherein saidthird field mirror (7625, . . . ) is an off-axis segment of a rotationalsymmetric reflective surface.
 11. The illumination system according toone of the claims 6 to 10, wherein said third field mirror (7625, . . .) is an on-axis segment of a toroidal reflective surface.
 12. Theillumination system according to one of the claims 6 to 11, wherein saidfirst (7027, . . . ), second (7023, . . . ) and third (7625, . . . )field mirrors are forming a non centered system.
 13. The illuminationsystem according to one of the claims 1 to 12, wherein said secondoptical component comprises an even number of normal incidence mirrorshaving incidence angles of less than 25°.
 14. The illumination systemaccording to one of the claims 1 to 13, wherein said first opticalcomponent further comprises a second optical element having a pluralityof second raster elements (7015, 7115, 7215, 7515, 7615, 7715, 8315,8415), wherein each of said plurality of first raster elements (7009, .. . ) corresponds to one of said plurality of second raster elements(7015, . . . ), and wherein said each of said plurality of first rasterelement (7009, . . . ) deflects a incoming ray bundle to saidcorresponding one of said plurality of second raster elements (7015, . .. ).
 15. The illumination system according to claim 14, wherein saidplurality of second raster elements (7015, . . . ) and said secondoptical component image said corresponding first raster elements (7009,. . . ) into said image plane (7029, . . . ).
 16. The illuminationsystem according to one of the claims 14 to 15, wherein said pluralityof second raster elements (7015, . . . ) are concave mirrors.
 17. Aprojection exposure apparatus for microlithography comprising: theillumination system of one of the claims 1 to 16 a reticle (8467) beinglocated at said image plane (7029, . . . ); a light-sensitive object(8473) on a support system (8475), and a projection objective (8471) toimage said reticle (8467) onto said light-sensitive object (8473).